There is a risk apparently of being a Lacto Ovo vegetarian, in that a deficiency of DHA (Docosahexaenoic acid) found in fish, can lead to Parkinsons disease: unless this Omega 3 is sourced from Algae.
This happened to Dr H M Shelton in his twilight years: a combination of many years of overwork and the erosion of his myelin sheath.
Flaxseed oil of course provides ALA which the body converts into DHA but apparently in insufficient quantities.
VITAMIN B12? From a previous post............
B12 is sourced in miniscule amounts from plants and is secured primarily from that produced within the body.
The stomach secretes a substance called "intrinsic factor", which transports the B12 created by the bacterial flora in our intestines.
Putrefaction/fermentation from improperly combined foods hampers this secretion of "intrinsic factor" in the stomach, and retards the production of B12 resulting in this deficiency.
Our actual need for Vitamin B12 though is so minute that it is measured in micrograms (millionth of a gram)or nanograms (billionth of a gram). One milligram of B12 will last over two years, and healthy individuals usually carry around a five-year supply.